This invention relates to an antilock system for a vehicle having a power-assisted hydraulic multiple-circuit brake system, including a control cylinder that is developed as a tandem master cylinder and can be operated by a pedal. A primary output pressure space of said control cylinder, that is delimited by the primary piston and a secondary floating piston, is connected via a pressure modulator to a brake circuit that can be statically acted upon by pressure. The brake circuit is able to be subjected to an antilock control operatin according to the principle of volume expansion.
In the case of a known antilock system of this type, one pressure modulator is provided for each of the total of four wheel brakes of the vehicle. The output pressure of one or the other output pressure space of a tandem master cylinder, that is provided as a brake control device and can be pedal-operated via a hydraulic or pneumatic brake-power booster unit of conventional construction, can be coupled into the connected wheel brake via the pressure modulars. Each of these pressure modulators has a modulator piston with two flanges that, in a pressure sealed manner, can be slid in a housing. The two flanges being connected with one another by means of a piston rod that is guided through a central bore of a partition of the modulator housing so that it can be slid in a pressure-sealed way. The output pressure space of the respective modulator, to which the assigned wheel brake is connected, is delimited by the other piston and the partition. The input pressure space of the modulator, that is connected to one of the control output pressure spaces of the tandem master cylinder, is delimited by the other piston flange and this partition. By means of the admission of pressure to the input pressure space, the modulator piston is shifted in the sense of a reduction of the output pressure space, causing brake pressure to build up in the connected wheel brake.
Two each of these modulators are housed in a joint housing part in such a way that the flanges of their modulator pistons, that in each case on one side delimit the output pressure space, jointly delimit a counterpressure space in axial direction. By the admission of pressure to the counter pressure space, during a control phase of the antilock system, the modulator pistons shifts in the sense of an enlargement of the volumes of the output pressure spaces of the modulators and results in a reduction of pressure in the connected wheel brake. Into the two counterpressure spaces of this pressure modulating arrangement, controlled via electromagnetic valves, the high output pressure of a pressure accumulator can be coupled simultaneously or alternately. The pressure accumulator, via an accumulator charge pump, is constantly maintained in a charged state. The pressure-reduction control valves are controlled in a conventional by pressure-reduction control signals of an electronic control unit. These signals are provided by a processing of wheel-speed-proportional output signals of each of the wheel-speed sensors individually assigned to the vehicle wheels.
The main disadvantage in the case of the known antilock system is that the modulator pistons in a pressure-reduction control phase must be shifted against a high pressure in the respective modulator input pressure space generated with the participation of the brake-power booster unit. Thus the accumulator charge pump must be designed for a very high nominal capacity, the typical value of which is 200 w. However, a hydraulic pump, that is designed for such a high nominal capacity and output pressures around 200 bar, not only requires a relatively large overall space but is also very costly. Therefore it contributes decisively to the costs to be spent for the known antilock system. The electric wiring of a vehicle equipped with the known antilock system with the high required nominal capacity of the accumulator charge pump, must be designed for a correspondingly higher output power.
It is therefore the objective of the invention to provide an antilock system of the initially mentioned type that, while it also has a simple construction and good operational reliability, on the whole can operate with a charge pump that is designed for a clearly lower nominal capacity, for a pressure accumulator that is utilized as an auxiliary pressure source.
This and other objectives are achieved by the following:
Accordingly, a tandem master cylinder with output modulator is provided as the brake control device that has an integrated hydraulic brake-power booster unit. The brake-power boosting takes place by the admission of an auxiliary pressure that is proportional to the pedal force and is derived from the output pressure of a pressure accumulator into a booster annulus. In a pressure-reduction phase of the antilock control, this annulus is blocked off with respect to the output of the proportional control valve and at the same time pressure is removed from the annulus into the brake fluid storage tank. This admission of pressure to a counterpressure space of the modulator will shift the modulator piston only against the pedal force applied via a plunger piston of the primary piston of the tandem master cylinder, i.e., against a considerably lower actuating force than in the case of a normal actuating of the tandem master cylinder. Thus, the accumulator charge pump, compared with the known antilock system, may be designed for about half the nominal capacity resulting in a considerable reduction of manufacturing costs. The pressure accumulator can be utilized also as an auxiliary power source for the brake-power boosting resulting in a further constructional simplification.
A second pressure modulator is formed for a second brake circuit of the vehicle that can statically be acted upon by pressure and consists of a counterpiston arranged in an extension of the housing of the tandem master cylinder. The secondary pistons of the tandem master cylinder can be shifted by means of the counterpiston. Thus the further constructional simplification is achieved of the hydraulic control part comprising the brake control device and the pressure modulators, of the antilock system and of the brake system as a whole.
To achieve a finely graduated antilock control individually controlled valves are provided for the booster annulus and the counterpiston. First, the increase of the brake pressure is reduced by the cancelling of the brake-power boosting. Finally, the brake pressure is further reduced by the admission of pressure to the counterpiston of the pressure modulator. Further control is achieved by three-position electro-magnetic valves for increasing, decreasing and maintaining the control pressure of the respective pressure spaces during normal and antilock braking.
The maintaining function can be provided using a two-position valve by sequentially alternating between the increasing and decreasing control pressure positions.
In addition to the brake circuits that are statically acted upon by pressure and that, are subjected to antilock control via pressure modulators, a pressure circuit is provided that can be dynamically acted upon by pressure, i.e., by the output pressure of the proportional control valve. This third brake circuit, independently of the other two brake circuits that themselves can be controlled independently from one another, is controllable according to the drain principle. The additional costs that are required for the realizing of a third controllable brake circuit are minimal.
The output pressure of the proportional control valve can also be utilized for the admission of pressure to the counterpiston of at least one of the pressure modulators.
Other details and characteristics of the invention are found in the following description of special embodiments by means of the drawing.